* ifioctl() calls (aka via ifconfig) make various assumptions about the
stability of certain data structures and can panic otherwise. Concurrent
calls to ifioctl via ifconfig can create inconsistencies in the ifnet
structure.

* Rearrange the ifioctl() code into a fall-through modem, add a struct mtx
to the ifnet structure, and acquire and release the mutex in ifioctl()
to enforce stability relative to concurrent ifconfig/ioctl commands
issued on the interface.